1. Field of the Invention
This invention relates generally to the field of modulators for Phase Amplitude Modulation (such as QAM or QPSK) transmission signal generation. More particularly, the invention provides a Phase-Amplitude modulator using a phase lock loop (PLL) to drive the power amplifier (PA) with phase and amplitude provided by either quadrature or polar input signals resulting in a low-cost, high power efficiency transmission solution for wireless communications.
2. Description of Related Art
As early as the 1950's, an “Envelop Elimination & Restoration” scheme was practiced in transmitter technology, mainly to raise the RF power amplifier efficiency. This is described in detail by Leenaerts, et al (Circuit Design for RF Transceivers, ISBN 0-7923-7551-3). The scheme, illustrated in FIG. 1, provides an alternative to traditional methods for adding the amplitude information at the power amplifier stage.
Traditional Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) transmitters consist of a quadrature I/Q modulator, depicted by FIG. 2, an up-converter and a linear RF power amplifier. A highly linear RF power amplifier is required, and a hi-order, low-loss band-pass filter is needed before the antenna to remove the side band signals as a result of the up-conversion. Both components are expensive, and both cause low power efficiency.
It is, therefore, desirable to provide modulation devices that could result in lower cost and higher power efficiency transmitters for QPSK and QAM signals for wireless applications.